Endosymbiosis CSF 2

16S rRNA::A component of the small subunit of the ribosome in bacteria and archaea. Its gene sequence is highly conserved and used as a molecular clock to infer evolutionary relationships and define bacterial species.

Alphaproteobacteria::A specific class of bacteria. Strong genomic evidence indicates that mitochondria are descended from an ancestor within this group.

Anaerobic::An organism that does not require oxygen for growth. Many ancient Archaea were anaerobes.

Aerobic::An organism that requires oxygen for metabolism.

Apoptosis::Programmed cell death. Mitochondria play a central role in initiating the intrinsic pathway of apoptosis.

Archaea::One of the three domains of life, single-celled prokaryotes often known for thriving in extreme environments (extremophiles). Genetically, eukaryotes are more closely related to Archaea than to Bacteria.

Bacteria::One of the three domains of life, single-celled prokaryotes.

Chloroplast::The photosynthetic organelle in plants and algae. It evolved from an endosymbiotic cyanobacterium.

Cristae::The folds of the inner mitochondrial membrane. This structure dramatically increases the surface area for ATP production via oxidative phosphorylation.

Endosymbiotic Theory::The well-supported scientific theory that certain organelles (mitochondria and chloroplasts) originated as free-living bacteria that were engulfed by an ancestral archaeal cell and evolved into a symbiotic relationship.

Eukaryote::An organism whose cells have a membrane-bound nucleus and other organelles. All complex life (plants, animals, fungi, protists) are eukaryotes.

Extremophiles::Organisms that thrive in physically or geochemically extreme conditions. Includes acidophiles, alkaliphiles, halophiles, psychrophiles, and thermophiles.

Facultative Anaerobe::An organism that can make ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation or anaerobic respiration if oxygen is absent.

Great Oxygenation Event (GOE)::A period around 2.4 billion years ago when cyanobacteria began producing significant amounts of oxygen via photosynthesis, leading to a dramatic rise in atmospheric oxygen. This was toxic to many anaerobes.

Hydrogen Hypothesis::Proposes that an anaerobic, hydrogen-dependent archaeon (a methanogen) engulfed a hydrogen-producing facultative anaerobic bacterium (an alphaproteobacterium). The symbiosis was based on a metabolic trade of hydrogen for nutrients, before the GOE.

Hydrogenosome::A membrane-bound organelle in some anaerobic eukaryotes that produces ATP and hydrogen. It is considered a highly modified mitochondrion that lost its genome. Supports the Hydrogen Hypothesis.

Intermembrane Space::The compartment between the inner and outer mitochondrial membranes. Crucial for generating the proton gradient used for ATP synthesis.

Matrix::The innermost compartment of the mitochondrion, enclosed by the inner membrane. It is where the TCA (Krebs) cycle occurs and contains the mitochondrial DNA and ribosomes.

Methanogen::A type of archaea that produces methane as a metabolic byproduct. They often use hydrogen and carbon dioxide.

Mitochondrion (pl. Mitochondria)::A double-membraned organelle found in most eukaryotes, responsible for producing ATP through aerobic respiration. It is the descendant of an endosymbiotic alphaproteobacterium.

Mitogenome::The genome of the mitochondrion. It is a small, circular DNA molecule, a relic of its bacterial past.

Nitroplast::A recently discovered (2024) nitrogen-fixing organelle in the marine alga Braarudosphaera bigelowii. It evolved from a cyanobacterium and is the fourth known instance of primary endosymbiosis.

Oxidative Phosphorylation (OXPHOS)::The process of ATP production using the electron transport chain and ATP synthase, which relies on the proton gradient across the inner mitochondrial membrane.

Ox-Tox (Oxygen Toxicity) Hypothesis::The traditional hypothesis for endosymbiosis. It proposes that an archaeon engulfed an aerobic bacterium to detoxify oxygen (a byproduct of the GOE) and, in return, gained efficient ATP production.

Prokaryote::A single-celled organism that lacks a membrane-bound nucleus and other organelles. Includes Bacteria and Archaea.

Reactive Oxygen Species (ROS)::Chemically reactive molecules containing oxygen, a natural byproduct of mitochondrial metabolism. At high levels, they cause cellular damage, but they also act as important signaling molecules.

Scientific Theory::A well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment (e.g., Cell Theory, Evolution, Endosymbiosis). It is not a "hunch" or a "guess."

TCA Cycle::(Tricarboxylic Acid Cycle/Krebs Cycle/Citric Acid Cycle) A series of chemical reactions used in aerobic organisms to generate energy through the oxidation of acetyl-CoA. Occurs in the mitochondrial matrix.